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Useful Notes / Tyrannosaurus rex

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"Sue" on display at the Field Museum of Natural History in Chicago, Illinois. Photographed by Evolutionnumber9 and licensed under CC BY-SA 4.0.

There's no doubt that Tyrannosaurus rex is, by far, the most famous of the stock dinosaurs, seen as both the most majestic and most terrifying of them. Let's learn a more about this incredible animal, shall we?

  • Name: Whereas most dinosaurs are known outside paleontological circles only by their genus, T. rex is known by its full binomium. Tyrannosaurus, the genus name, comes from the Greek tyrannos, meaning "tyrant", and sauros, meaning "lizard". Rex, the specific name, meanwhile, is Latin for "king", therefore the whole name translates to "tyrant lizard king". As per the rules of binomial nomenclature, the genus name should be spelled with a capital T, whereas the species name should be spelled with a lowercase r. And the correct abbreviation of the name is T. rex, not T-rex and especially not T-Rex. Early fossils were described under the names Dynamosaurus imperiosus and Manospondylus gigas — both of them are now considered invalid synonyms. A number of other names have also become invalid synonyms of Tyrannosaurus over the years, including Stygivenator and Dinotyrannus. One particular synonym, Nanotyrannus, was long believed to be a dwarf T. rex relative for its slender frame and narrower snout before scientists realized that juvenile T. rex simply lacked the heavy skulls and robust bodies of the grown-ups. As of 2022, it's been suggested that the oldest specimens of Tyrannosaurus belonged to a separate species called T. imperator ("tyrant lizard emperor") that then evolved into T. rex and another, more gracile species called T. regina ("tyrant lizard queen"), but this is not widely accepted. Note  T. imperator would be the species whose famous giant specimen named "Sue" belongs to.
  • Discovery: The first T. rex fossils were found in 1874 and were believed to belong to a giant ornithomimid or a ceratopsid. The species was officially described by Henry Farfield Osborn in 1905, based on a partial skeleton consisting of 34 bones, found by Barnum Brown in Hell Creek, Montana. Osborn described another fossil of a large carnivore, found in Wyoming, as Dynamosaurus imperiosus ("ruling power lizard"), but then realized that the two belonged to the same species. The most complete T. rex skeletons were found in 1990 and 1992, and were dubbed as "Sue" and "Stan". These two fossils helped us get a much more accurate image of what the species was like.
  • Classification: It was believed for most of the 20th century that Tyrannosaurus was closely related to other giant carnivorous dinosaurs, such as Allosaurus and Megalosaurus, as part of a group of dinosaurs called Carnosauria. However, some scientists instead believed T. rex was part of the group Coelurosauria, which includes many small, feathered dinosaurs such as Ornithomimus, Compsognathus, Velociraptor, and Archaeopteryx as well as modern birds — a belief that was eventually validated by studies in the early 1990s. Within Coelurosauria, T. rex was part of the family Tyrannosauridae. Other tyrannosaurids include Tarbosaurus (T. rex's closest relative; sometimes considered an Asian Tyrannosaurus species), Albertosaurus, Gorgosaurus (sometimes synonymized with Albertosaurus), Daspletosaurus (formerly considered T. rex's ancestor), Teratophoneus, Lythronax, Dynamoterror, Nanuqsaurus, Zhuchengtyrannus, Alioramus, and Qianzhousaurus (sometimes synonymized with Alioramus). They were part of the superfamily Tyrannosauroidea, which also includes Alectrosaurus, Appalachiosaurus, Bistahieversor (those three sometimes considered true tyrannosaurids), Dryptosaurus, Suskityrannus, Moros, Timurlengia, Yutyrannus, Proceratosaurus, Eotyrannus, Stokesosaurus, Dilong, and Guanlong. Also possibly within Tyrannosauroidea are the megaraptorans (named for their most famous member Megaraptor), a group of theropods primarily found in the Southern Hemisphere and characterized by slender skulls and massive arms tipped with talon-like claws that were likely used in killing prey, though they may have instead been within Carnosauria.
  • Time period: T. rex lived at the very end of the Cretaceous period, 68-66 million years ago (also known as the Maastrichtian age), although a possible Tyrannosaurus lacrimal roughly 10 million years older was reported in 2006. It was among the few dinosaurs still around when the famous asteroid collision ended the Mesozoic era. Any depiction of T. rex earlier than that is, therefore, inaccurate.
  • Range: T. rex was an exclusively North American species. During the Late Cretaceous, North America was divided to two smaller continents by a shallow sea named the Western Interior Seaway; T. rex lived on the western continent, dubbed Laramidia. It was found throughout Laramidia, ranging from Alberta and Saskatchewan in the north to Texas and New Mexico in the south (and possibly even Sonora, Mexico). A closely related species, the aforementioned but smaller Tarbosaurus bataar, lived in East Asia at the same time. It is theorized that because of how closely related it was to its Asian counterpart, T. rex's ancestors likely originated from Asia and crossed over the Bering Strait land bridge into North America, eventually taking over most of Laramidia. Albertosaurus, which lived in Laramidia before this, became extinct shortly after T. rex appeared, lending credence to this theory.
  • Size: T. rex was famous for being one of the largest carnivorous dinosaurs, rivalling Giganotosaurus and Spinosaurus in length and likely being heavier than either. The "Sue" skeleton is 12.3 to 12.8 m long (the inaccuracy comes from a few missing vertebrae), about 4 m tall at the hip, and is estimated to weigh 8.4 to 14 metric tons (for comparison, that's 2 to 3 times the weight of an African bush elephant). It was also much bulkier in real life compared to the athletic, "cut"note  manner that fiction and older reconstructions tend to depict it with. As a result, it probably wasn't very fast (current estimates of its top speed generally agree on somewhere between 18-24 km/h) but given that most of its usual prey was also large and bulky, it didn't have to be.
  • Posture: Early restorations depicted T. rex in a kangaroo-like tripod posture, dragging its long tail on the ground like Godzilla. However, thanks to more complete skeletons, now we know that it held its body horizontally, balancing its body with its tail.
  • Big head: T. rex had a massive head, even in comparison to other carnivorous dinosaurs. Its mouth was full of sharp teeth, up to 20 cm (8 inches) long, sometimes dubbed "killer bananas" because of their size and shape. Its bite force is estimated to be about 8,000 pounds, stronger than any other known land animal, which it needed to crush the bones of the large, often armored dinosaurs it ate.
  • Puny arms: One of the most iconic, and most ridiculed, traits of T. rex is its tiny arms (though many theropods that had even smaller arms). They were two-fingered with sharp claws on them, and the palms faced inward (rather than downward, as often erroneously depicted). The reason for the arms' small size is mainly practicality; large arms would have gotten in the way of the Tyrannosaurus's bite, which is theorized to have been the most powerful bite of all dinosaurs, hence the need to have small arms where other predators would have clawing weapons. Additionally, in spite of their size, the arm bones show signs of large muscle attachment and thus, they were very strong and capable of lifting up to 200 pounds (90 kilograms). Because of this, T. rex might have used them to hold onto struggling prey while it dispatched it with its jaws. In addition, they could have also been used to help lift T. rex up from a sleeping position when it was waking up, hold onto a mate while mating, or even to pick up its offspring if the situation called for it. Interestingly, baby and juvenile Tyrannosaurus had slightly larger arms compared to their total body size.
  • Hunter or scavenger: Though the T. rex is typically portrayed as a hunter in media, there was an infamous debate among paleontologists as to whether or not T. rex was actually a scavenger instead, popularized by paleontologist and Jurassic Park dinosaur consultant Jack Horner. While the idea of a large, bulky carnivore solely surviving on rotting carrion is just too ridiculous to be taken seriously (due to the amount of food needed to sustain its massive size), it is still possible that large, adult Tyrannosaurus would hunt less and basically Kill Steal the hunts of other carnivores and younger T. rex; the highly advanced olfactory sense of the creature allowed it to sniff out carrion from miles away, and a roar (but see below) would have definitely helped scare off smaller dinosaurs from a kill. Of course, this is something that even modern land predators, like tigers, do when they reach old age. Due to an adult's size and strength, however, it's also very possible that it simply fought heavily armored animals like Triceratops or Ankylosaurus and won, while also being able to overpower behemoths like fully-grown Edmontosaurus and subadult Alamosaurus. Indeed, fossils of all these dinosaurs have been found with bite marks attributable to T. rex, several of which also show signs of healing, indicating the herbivores survived the encounter. On the other hand, younger, smaller Tyrannosaurus, with their slenderer bodies and narrower snouts, hunted faster-moving and more delicate mid-sized animals like Ornithomimus, Thescelosaurus, Leptoceratops, and Pachycephalosaurus. Even in that case, a roar could've been helpful; even the most active predators are unlikely to pass up the opportunity to steal a free meal if they can, and a Tyrannosaurus might well have needed to threaten off other dinos (including other Tyrannosaurus) while eating. Bite marks on T. rex bones attributed to other T. rex also suggest it may have even been cannibalistic.
  • Feathers or scales: Historically, T. rex was portrayed with lizard- or crocodile-like scaly skin. As many dinosaur species were discovered to be covered in feathers, including a tyrannosaur relative, Yutyrannus, it was suggested that T. rex could also have been feathered, basically looking like a giant toothy bird. However, fossilized skin impressions of T. rex and other, closely related tyrannosaurs show that most of its body was, indeed, scaly (or at least, extremely sparsely feathered); the only place that was potentially fully feathered is its back, similar to the mane of a lion. As T. rex was a large animal living in warm climate, it likely did not need the extra insulation from a thick coat of fluff, just as similarly-sized mammals like rhinoceroses and elephants are sparsely haired. Some still speculate that it had downy feathers as a hatchling, when it was still small enough to need insulation, and eventually lost these as it reached a certain age and size, much like a baby penguin molting away its birth feathers as it becomes an adolescent.
  • Sound: T. rex is typically depicted in media with a Mighty Roar; ever since Jurassic Park, everyone "knows" what that roar sounded like. However, there is actually little evidence that T. rex could roar, since it lacked the vocal organ to do so (it may have had a larynx like birds do, but it doesn't produce sounds). If it vocalized at all (which is still uncertain), it may have produced low-pitched, rumbling or bellowing sounds, similar to crocodilians or large flightless birds (which would've still sounded pretty damned impressive, but possibly a bit underwhelming for those used to the movies), as well as hissing. Imagine the sound an alligator makes, then imagine if that alligator was 40 feet long and weighed over 8 tonnes. The vibrations produced by a communicating T. rex may have even been deep and powerful enough to have been felt when they rippled through the air, possibly even creating small tremors in the earth.
  • Senses: T. rex had extraordinary senses of smell and hearing. Analysis of the braincase in fossilized skulls shows that it had large olfactory bulbs and a long cochlear duct capable of receiving low-frequency sounds. These traits would have been advantageous both as a predator (hunting prey) and as a scavenger (finding carrion from a great distance or detecting approaching rivals). However, contrary to what Steven Spielberg and Michael Crichton would have you believe, T. rex also had excellent eyesight that would've let the creature spot objects from as far as six kilometres with thirteen times the detail. It had large eyes facing forward connecting to big optical lobes, and a relatively narrow snout, allowing for binocular vision. This is probably T. rex's most unique trait, as almost no other dinosaur had vision like this. This ability supports the idea that T. rex was a hunter, as binocular vision is beneficial when chasing prey.
  • Intelligence: Most early depictions of T. rex depict them as solitary brutes with little in the way of brainpower or social ability. More recent studies have shown that T. rex had a bigger brain-to-body ratio than previously thought, being smarter than earlier species of giant theropods. Given its adaptations and how its common prey items included some heavily armored and highly social animals, it's likely T. rex would need the intelligence to be able take down its prey with different strategies and know when to throw in the towel and eat a pre-killed carcass instead. Some scientists have also proposed that T. rex sometimes hunted in packs, but the idea of such coordinated behavior in predatory dinosaurs is controversial, as neither birds nor crocodiles hunt in packs. That is, until captive Cuban crocodiles were observed to be behaving in a way that could only be called "pack hunting", which further confuses this issue. Even more recently, studies have suggested that T. rex may have had an intelligence similar to wolves, or even baboons, which would have made it one of, if not the, smartest non-avian dinosaurs.
  • Neighbors: In art and fiction, T. rex is often portrayed living alongside dinosaurs it didn't coexist with in reality. The most egregious cases show it alongside Jurassic dinosaurs like Stegosaurus (Fantasia, for example), but even authors who try to do their homework often slip up by showing T. rex coexisting with Late Cretaceous North American dinosaurs that lived before it during the Campanian age, like Parasaurolophus, Corythosaurus, Styracosaurus, Euoplocephalus, and Stenonychosaurus (aka Troodon). Dinosaurs that did live alongside T. rex 68-66 million years ago included Triceratops, Ankylosaurus, Edmontosaurus, Alamosaurus, Dakotaraptor, Pachycephalosaurus, Ornithomimus, Torosaurus, Denversaurus, Thescelosaurus, Leptoceratops, Anzu, Acheroraptor, Pectinodon, and Trierarchuncus. No other species of tyrannosaur coexisted with T. rex for most of its existence, with the last one, Albertosaurus, dying out 68 million years ago— this made it the undisputed apex predator of its environment and the very last of the North American tyrannosaurs. Also found in T. rex's habitat were the giant pterosaur Quetzalcoatlus and the primitive amphibious reptile Champsosaurus, as well as small mammals (including early relatives of marsupials and primates), true birds (including early relatives of ducks), crocodiles, turtles, snakes & lizards (including early boas and relatives of the Gila monster), many kinds of amphibians (including giant siren salamanders), and a variety of fish (such as freshwater sharks & rays, sturgeons, gars, paddlefish, and bowfins). Many of these smaller animals are still found in North America today.
  • Environment: The prehistoric North American range of T. rex was divided between a swampy subtropical forest to the north and arid inland plains to the south. These environments supported different faunas. The former, known as the Lancian fauna and most famously represented by the Hell Creek Formation of Montana, was dominated by Triceratops, Edmontosaurus, and Ankylosaurus, while the latter, the less-studied Sanjuanian fauna (known primarily from the Ojo Alamo Formation of New Mexico), was dominated by Alamosaurus, Torosaurus, and Quetzalcoatlus. Most of T. rex's contemporaries mentioned above are Lancian, with many having Sanjuanian counterparts — Ojoraptorosaurus instead of Anzu, Dineobellator instead of Acheroraptor, Glyptodontopelta instead of Ankylosaurus, and Kritosaurus instead of Edmontosaurus. These faunal differences have led some scientists to suggest the southern T. rex population actually represents a second genus of tyrannosaurid named Alamotyrannus. For now however, it appears that T. rex was one of the most widespread and adaptable large carnivorous dinosaurs to have ever lived.